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Past research has indicated that people of lower socioeconomic status are more prone to asthma, allergies and other inflammatory disorders. Now, a new study suggests this may be because such individuals are more likely to live in urban areas, reducing their exposure to "healthy microbes" in rural settings.

The research team, including Christopher Lowry of the Department of Integrated Physiology at the University of Colorado Boulder, published their findings in the journal Clinical & Experimental Immunology.

It is well known that the immune system produces an inflammatory response to fight off infection. In a healthy immune system, this inflammation reduces once the infection is gone. But if immune system function is impaired, inflammation persists. This can lead to numerous health problems, such as irritable bowel syndrome (IBS), asthma and allergies.

"The rise of chronic inflammation and these associated disorders, especially among people living in the cities of developed countries, is troubling," says Lowry.

The hygiene hypothesis

According to the background of the study, researchers have suggested that populations in richer Western countries are more prone to chronic inflammation because they have become too clean - what is known as the "hygiene hypothesis."

Exposure to microbes in rural environments may reduce the risk of asthma and allergies, according to new research.

In detail, this hypothesis explores the idea that some microbes and infections combine to abolish inflammation in the immune system and that reducing exposure to these may lead to health problems.

However, the researchers say that this idea can cause misunderstanding when applied to low-income communities, who tend to experience both disorders associated with chronic inflammation and infections caused by germs.

The team says they believe that microbes and some infections can stop the immune system from triggering inflammation when it is not needed - a process that is apparent in asthma attacks and allergic reactions. But they note that "modern diseases" that we can pick up from crowded spaces, such as school and work, do not have this anti-inflammatory effect.

Evolution of the human immune system

Explaining this further, the team talks about the evolution of the human immune system.

In the past, the researchers say that humans were mainly exposed to microbes and infections in three ways. They were either passed from our mothers or family members, through nonpathogenic microbes from the environment or through chronic infections, such as helminths - parasitic worms that reside in the blood and gut.

They say that in order to prevent the immune system from triggering inflammation, these "old infections" evolved so they could remain in the body for long periods. However, modern diseases, such as measles or chicken pox, do cause an inflammatory response of the immune system.

From this, the investigators believe that microbes and old infections - which they collectively refer to as "old friends" - are important in protecting against inflammatory disorders. But because these old infections are not present in the developed world, it is important that humans are exposed to environmental microbes, such as those found in rural settings.

The team says this idea may explain why individuals of a low socioeconomic status are more prone to inflammatory disorders - they cannot afford to live in rural settings, therefore they have reduced exposure to rural microbes.

As such, the team recommends that individuals who live in urban areas should focus on reducing exposure to modern diseases by increasing hygiene levels, as well as increasing exposure to greener environments.

They conclude:

"We hope that these insights will help to focus attention on the need for increased hygiene, coupled with innovative design for homes and sustainable cities that promote appropriate microbial exposures.

Meanwhile there is enormous need for greater understanding of the relationship between organisms in the natural environment and those that colonize us so that we can optimize the design of urban green spaces."

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